Thanks for your input Stebb, I tried to argue those very points with the
dealers.
I've had seizing calipers and brakes running hot before. But the brakes are not running hot or pulling. The
dealer said they checked for seizing brakes, driveshafts etc. I too think that the brakes etc. are OK, there doesn't seem to be a
problem with rolling resistance and the engine doesn't "feel" tight. At very low speeds (steady 30 in 4th) it appears free and is reasonably economical. But a small amount of additional work knocks it flat. 5th gear cannot return good economy even at a steady speed. And accelerating, changing gear or the slightest incline kills the mpg. I've tried different driving styles, speeds and revs but nothing works.
I hoped that the clicking/crackling noise the engine makes when under load and the occasional air current noise were the cause of the
problem and they could be repaired but apparently the noises are normal.
It's impossible to "guess" what an engine fault might be. If the computer and the diagnostics can't identify it then the
dealer has to say it's all ok. I think it's the Technical Unit rather than a
dealer that needs to properly investigate it. But as long as the engine is running ok, low mpg is not regarded as a
problem!